Boilers in current use include raw water boilers using raw water such as industrial water, well water or the like, soft water boilers and pure water boilers. In many cases, the temperature of the water in these boilers is set at 110.degree. C. to 400.degree. C. under various pressures.
So-called pure water boilers use, as boiler water, water having an electric conductivity of 1.0 .mu.S/cm or less such as pure water, ion-exchanged water or the like, from which ions such as alkali metal ions, alkaline earth metal ions (hardness causing components), sulfate ions, chlorine ions and the like are substantially removed.
In such pure water boilers, carbonate ions which are inevitably present in water due to the dissolution of carbon dioxide gas contained in air and oxygen dissolved in the water will cause corrosion of a ferrous metal. Corrosion such as pitting corrosion can easily occur and result in critical accidents.
On the other hand, soft water boilers employ soft water which is substantially free from hardness causing components (1 mg/l or less), but which contain anionic components such as sulfate ions, chloride ions or the like. The presence of the anionic components and dissolved oxygen in soft water cause corrosion of a ferrous metal in the system, especially pitting corrosion.
Therefore, pure water boilers and soft water boilers, employ a method of preventing the corrosion of the ferrous metal, specifically, pitting corrosion thereof, in which pure water or soft water is passed through a deaerator and then deoxidized by an oxygen scavenger such as a sulfite, hydrazine or the like, which is added to the water, and a phosphate anticorrosive agent which is then added to the deoxidized water, and as occasion demands, an alkali agent is added to the water so as to adjust the pH to 10 to 12. This method is called deoxidation/alkali treatment method.
However, the above-mentioned method requires troublesome deoxidation treatment and is insufficient to prevent the corrosion of a ferrous metal.
As a method of preventing or controlling corrosion of a ferrous metal in the water system of a pure water boiler, without conducting deoxidation, the inventors of this application propose a method in which a volatile amine and an aliphatic hydroxycarboxylic acid such as lactic acid, citric acid, malic acid or the like are added to pure water (Japanese Patent Laid-Open No. 58-96881).
As anticorrosive agents which can prevent or suppress the corrosion of a ferrous metal in the water system of pure water or soft water boilers without conducting deoxidation, the inventors also propose the following agents:
a four-ingredient anticorrosive agent containing as effective ingredients, (a) at least one compound selected from the group consisting of molybdic acid, tungstic acid and nitrous acid and an alkali salt thereof, (b) an aliphatic hydroxycarboxylic acid or an alkali salt thereof, (c) a compound which can easily release metal ions in water, and (d) a polymer or copolymer of an olefinic compound which has molecular weight of 500 to 100,000 and is soluble in water (Japanese Patent Publication No. 62-57715: U.S. Pat. No. 4,512,552); and
a five-ingredient anticorrosive agent containing the above four ingredients and a monoamine having 2 to 8 carbon atoms (Japanese Patent Publication No. 62-27157).
In recent years, however, the capacity of a water purifying apparatus for supplying pure water, ion-exchanged water, soft water or the like tends to become insufficient with an increase in evaporation and heat transfer load of a boiler, as compared with the water-holding capacity thereof.
As a result, ion ingredients such as alkali metal ions, sulfate ions, chloride ions or the like are added into the water system of a pure water boiler so that the electric conductivity of the water supplied to the pure water boiler exceeds 1 .mu.S/cm. Further, the water hardness of a soft water boiler is frequently increased to 20 mg/l or more by addition of such hardness causing components to the water of a soft water boiler (hardness leak) under circulation.
Moreover, a silicate or iron component is also frequently incorporated in the water due to a limitative capacity of a water purifying apparatus, water softener or an ion-exchange resin.
When the method disclosed in Japanese Patent Laid-Open No. 58-96881 is performed in the water system of a pure water boiler having such various impurities incorporated therein, the anticorrosive effect is deteriorated. This tendency is remarkable when the electric conductivity of the water supplied to the pure water boiler exceeds 1.0 .mu.S/cm, and when the content of the ferrous components exceeds 0.1 mg/l.
On the other hand, the addition of the anticorrosive agent, which is disclosed in Japanese Patent Publication No. 62-27157 or 62-57715, to the water system of a soft water boiler in which the hardness components are included, causes a nonuniform deposition of a black anticorrosive film due to the adhesion of scales to the surface of a ferrous metal. As a result, the anticorrosive effect on the ferrous metal is sometimes made insufficient. This tendency is remarkable when the content of hardness components exceeds 20 mg/l.
Although the polyamine represented by the formula (I) described below and used in the present invention has been known as a water treatment agent for boilers (Japanese Patent Publication No. 50-35601), the use of such a polyamine in combination with other compounds, as in the present invention, and the effect thereof have not been known.